Device for operating a gas discharge lamp

ABSTRACT

A device for operating a gas discharge lamp ( 13 ) is proposed, having a power supply circuit which makes available an alternating voltage (U L ) or an alternating current (I L ) of a predetermined period duration (T P ) for the supply of the gas discharge lamp ( 13 ) with predetermined electrical power. During an intended reduction of the mean lamp rating compared to the operation at normal rating, an increase of the instantaneous power is provided within a half period duration (T P /2) prior to the pole change of the alternating voltage (U L ) or alternating current (I L ). Preferably, the duration of the increase of the instantaneous power is a function of the period duration (T P ). The device according to the invention for operating the gas discharge lamp ( 13 ) largely prevents the arc comprised in the gas discharge lamp ( 13 ) from going out during the pole-changing process and therefore ensures reignition following the pole-changing process. A preferred application of the device according to the invention is a gas discharge lamp ( 13 ) which is installed in a motor vehicle.

BACKGROUND OF THE INVENTION

The invention is based on a device for operating a gas discharge lamphaving a power supply circuit which makes available an alternatingvoltage or an alternating current of a predetermined period duration forthe supply of the gas discharge lamp with a predetermined power. FromDE-A 37 15 162.2, a circuit arrangement for operating a gas dischargelamp is known comprising a voltage transformer that converts the energymade available by a direct current source to an alternating voltage ofappropriate level and frequency. At control inputs, the voltagetransformer receives control signals derived from the lamp voltage andfrom the lamp current to supply the gas discharge lamp with apredetermined electrical power. During the pole-changing process of thealternating voltage, an instantaneous power reduction occurs in the gasdischarge lamp, which power reduction can lead to a short-time reductionof the arc. A sufficiently high operating frequency, which may be, forexample, at a few 100 Hertz, ensures that the arc does not go outentirely during the short current zero. The required voltage after thepole-changing process for the maintenance of the arc during operation atnormal rating corresponds to the normal burning voltage of the gasdischarge lamp. A reduction of the electrical power supplied to the gasdischarge lamp results in a total interruption of the arc during thepole-changing process. The power supply circuit must be able to come upwith the necessary reignition voltage after the pole-changing process. Areduction of the electrical power supplied to the gas discharge lamp istherefore possible to the same extent that the power supply circuit isable to come up with the reignition voltage.

It is the object of the invention to provide a device for operating agas discharge lamp which ensures a reliable operation of the gasdischarge lamp in an operating phase with reduced power.

SUMMARY AND ADVANTAGES OF THE INVENTION

The above object generally is achieved according to the presentinvention by a device for operating a gas discharge lamp, including apower supply circuit which makes available an alternating voltage or analternating current of a predetermined period duration for the supply ofthe gas discharge lamp with a predetermined power, wherein upon adesired reduction of the mean lamp operating power rating compared tothe operation at normal power rating, an increase of the instantaneouspower is provided within a half period duration immediately prior to thepole change of the alternating voltage or alternating current suppliedto the lamp.

The device according to the invention offers the advantage that anincrease of the output voltage of an existing power supply circuit isnot necessary in order to accomplish a higher reignition voltage. Theinvention provides that, during a reduction of the mean lamp ratingcompared to the operation at normal rating, an increase of theinstantaneous power is provided within a half period durationimmediately prior to the pole change of the alternating voltage. Theshort-time increase of the instantaneous power prior to thepole-changing process has the effect that the necessary reignitionvoltage after the pole-changing process is not considerably highercompared to the operation at normal rating.

Advantageous modifications and configurations of the device according tothe invention ensue from the dependent claims.

An advantageous feature provides that the increase of the instantaneouspower prior to the pole-changing process corresponds to the power duringoperation at normal rating. In this design, a power reserve of the powersupply circuit is not necessary.

In another advantageous configuration, wherein the power supply circuitcan supply a power which is above the rated power, it is provided thatthe short-time instantaneous power is above the rated power. Thismeasure ensures a reignition of the gas discharge lamp after thepole-changing process in all conceivable operating conditions, alsotaking ageing of the gas discharge lamp into account.

According to an advantageous modification, it is provided that theincrease of the instantaneous power within a half period duration priorto the pole change of the alternating voltage is provided in each halfperiod of the alternating voltage. This measure further increases thereliability of the reignition of the gas discharge lamp after thepole-changing process.

A further advantageous modification provides that the increase of theinstantaneous power is implemented, in particular, by increasing thecurrent, with the current intensity being predetermined. The currentintensity determines the ionization conditions in the gas discharge lampand therefore is an essential characteristic quantity which is ofimportance particularly for a pole-changing process.

A modification of the device according to the invention provides thatthe increase of the instantaneous power takes place a predetermined timeprior to the pole-changing process, which time is a function of theperiod duration of the alternating voltage. This advantageousmodification allows an adaptation to a different operating frequency ofthe gas discharge lamp.

A further modification relates to the power supply circuit which ispreferably configured as voltage transformer supplying a direct voltageat its output, which direct voltage is converted to an alternatingvoltage in a subsequent bridge circuit in whose diagonal the gasdischarge lamp is disposed.

A feature of this modification provides that the voltage transformer isa DC/DC converter fed by a battery.

The modifications of the power supply circuit which are providedaccording to the invention allow the realization of a highly-efficientvoltage transformer which, internally, may have a considerably higherclock frequency than the alternating voltage that is supplied to the gasdischarge lamp. The comparatively low-frequency alternating voltagehaving a frequency of, for example, a few 100 Hertz, is generated in thebridge circuit.

The device according to the invention is particularly suited foroperating a metal-doped high-pressure gas discharge lamp which iscritical with respect to the need for reignition after the arc has goneout. A preferred application of the high-pressure gas discharge lamp,particularly of the metal-doped high-pressure gas discharge lamp, is theuse in a headlight of a motor vehicle.

Further advantageous modifications and features of the device accordingto the invention for operating a gas discharge lamp ensue from furtherdependent claims and from the following description.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 illustrates a block diagram of a device according to theinvention, and

FIG. 2 illustrates a signal pattern appearing in the block diagramaccording to FIG. 1 as a function of the time.

FIG. 1 shows a voltage transformer 10 which converts an electrical powersupplied by a power source 11, which has a voltage value U_(B), to acorresponding output-side power having a predetermined output voltageU_(A) or a predetermined output current I_(A). The output voltage U_(A)is supplied to a bridge circuit 12 at whose output a lamp voltage U_(L)appears which is provided to supply a gas discharge lamp 13. A lampcurrent flowing through the gas discharge lamp 13 is identified byI_(L).

The voltage transformer 10 is fed a control signal 14 made available bya power specification arrangement 15. The power specificationarrangement 15 determines the control signal 14 as a function of apredetermined power 16, the captured output current I_(A), the capturedoutput voltage U_(A) and a start signal 17. The start signal is suppliedto an arrangement 18 comprised in the power specification arrangement 15for the purpose of increasing the desired power value. The start signal17 is made available by a timer 20 included in a switching signalgenerator 19. A switching signal 21 generated by the switching signalgenerator 19 is fed to the bridge circuit 12.

FIG. 2 shows the lamp current I_(L) as a function of the time t. Thelamp current I_(L) has an at least approximately rectangular-shapedsignal pattern, with the period duration T_(P) being set. A firstcurrent level corresponds to a rated current I_(N), a second currentlevel corresponds to a reduced current I_(R) and a third current levelcorresponds to a pulse current I_(P). The pulse current I_(P) appearswithin a half period duration T_(P)/2 after a delay time T_(V).

The arrangement illustrated in FIG. 1 is explained in greater detail byway of the path of the lamp current I_(L) shown in FIG. 2 as a functionof the time t:

Together, the voltage transformer 10, the bridge circuit 12 as well asthe power specification arrangement 15 and the switching signalgenerator 19 form a power supply circuit for the gas discharge lamp 13.The voltage transformer 10 converts the power supplied by the powersource 11 to a predetermined output voltage U_(A) and/or a predeterminedoutput current I_(A). The power source 11 preferably is a battery withan impressed voltage U_(B). The voltage transformer 10 preferably is adirect voltage/direct voltage (DC-DC) converter which converts thevoltage U_(B), which is a direct voltage if a battery is the powersource 11, to a generally higher output voltage U_(A) which is also adirect voltage. The downstream bridge circuit 12 has the task ofconverting the direct voltage U_(A) to a lamp voltage U_(L) suitable forsupplying the gas discharge lamp 13. An alternating voltage is suitablewhich prevents material transport effects in the gas discharge lamp 13as well as uneven wear of the electrodes. The bridge circuit 12comprises, for example, four semiconductor power elements wired asH-bridge in whose diagonal the gas discharge lamp 13 is arranged. Thebridge circuit 12 is switched with the switching signal 21 in such amanner that the output voltage U_(A) of the voltage transformer 10 isapplied, alternating in polarity, to the two electrodes of the gasdischarge lamp 13. The frequency which is, for example, a few 100 Hertz,is determined by the switching signal 21. The configuration of thevoltage transformer as a DC-DC converter whose output voltage U_(A) isconverted with the bridge circuit 12 to an alternating voltage, offersthe advantage that the internal clock frequency of the DC-DC converter10 can be significantly higher than the frequency of the switchingsignal 21. The voltage transformer 10 can have an internal clockfrequency of a few 100 kilohertz.

The voltage transformer 10 can impress either the output voltage U_(A)or the output current I_(A) with the gas discharge lamp 13 as load. Therespectively other quantity is fixed by the internal resistance of thegas discharge lamp 13. Preferably, the output current I_(A) is impressedsince the gas discharge lamp 13 is designed for specific currents. Inits amount or value, the output current I_(A) corresponds to the lampcurrent I_(L). Likewise, the output voltage U_(A) and the lamp voltageU_(L) correspond with respect to their amounts.

For the predetermination of a specific power, the power specificationarrangement 15 is provided which emits a corresponding control signal 14to the voltage transformer 10. The power specification arrangement 15receives the power to be predetermined from the power specification 16as a desired value. The actual power value is determined from thecaptured output voltage U_(A) and the captured output current I_(A). Thepotentially necessary sensors are not indicated in detail in FIG. 1.

During the operation at normal rating of the gas discharge lamp 13, theamplitude of the lamp current I_(L) amounts to, for example, the valueof the rated current I_(N) indicated in FIG. 2. The lamp current I_(L)has a rectangular-shaped signal pattern having a period duration T_(P)which is predetermined by the switching signal 21. During thepole-changing process, during which the lamp current I_(L) and the lampvoltage U_(L) pass through zero, a more or less pronounced reduction ofthe arc appearing in the gas discharge lamp 13 occurs as a function ofthe prevailing operating data of the gas discharge lamp 13. In theextreme case, the arc may go out totally after the pole-changingprocess. During operation of the gas discharge lamp 13 at normal rating,in which, for example, the rated current I_(N) is flowing, there isgenerally no considerable increase of the reignition voltage after thepole-changing process. In order to ensure a reliable reignition of thelamp after a pole-changing process, even in a reduced power mode of thegas discharge lamp 13 during which, for example, the reduced currentI_(R) is flowing, it is provided according to the invention thatimmediately prior to the pole-changing process of the lamp voltageU_(L), an increase of the instantaneous power takes place within a halfperiod duration T_(P)/2. To increase the instantaneous power, forexample, the voltage or, in particular, the current can be increased. Inthe illustrated embodiment, the lamp current I_(L) is increased to thepulse current I_(P) compared to the reduced current I_(R) beforepole-changing. This measure reduces the reignition voltage after anextinction of the arc in the gas discharge lamp 13 which might haveoccurred following the pole-changing process. The pulse current I_(P)can be selected to be, for example, identical to the rated current I_(N)With this design, the voltage transformer 10 does not have to have anycurrent reserves. Preferably, the pulse current I_(P) is above the ratedcurrent I_(N) by a predetermined amount, with it being necessary thatthe voltage transformer 10 is able to supply the increased pulse currentI_(P). This measure accomplishes greater reliability. Furthermore, itmay be provided that, in each half period duration T_(P)/2, a short-timeincrease of the instantaneous power is provided immediately prior to thepole-changing process. This measure further increases the reliability ofreignition. The information as to the moment at which the instantaneouspower must be increased can be communicated to the power specificationarrangement 15 by the switching signal generator 19. The switchingsignal generator 19 has the information on the period duration T_(P) andthe condition of the pole-changing processes. The start signal 17 isemitted to the power specification arrangement, for example, a fixedlypredetermined amount of time prior to the pole-changing process. Thestart signal 17 effects an increase of the desired value via thearrangement for increasing the desired power value. The predeterminedtime may be fixed, for example, by the timer 20 in the switching signalgenerator 19. The timer 20 may be started, for example, by apole-changing process and emits the start signal 17 after thepredetermined delay time T_(V) has expired.

According to a preferred modification it is provided that the delay timeT_(V) depends on the period duration T_(P). Thus, the time duration forincreasing the instantaneous power by increasing from the reducedcurrent I_(R) to the pulse current I_(P) can be controlled with regardto time as a function of the period duration T_(P). This results in anadaptation to the operating behavior of the gas discharge lamp 13, withit being more probable that the arc will go out if the period durationT_(P) is made longer. The adaptation takes place such that, in case ofan increase of the period duration T_(P), the delay time T_(V) isincreased less than proportionally. This increases the period durationof the instantaneous power increase within the half period durationT_(P)/2.

A metal-doped high-pressure gas discharge lamp, for example, is providedas a gas discharge lamp 13. Such a gas discharge lamp is suited asheadlight lamp which is used, for example, in a motor vehicle.

What is claimed is:
 1. A device for operating a gas discharge lamp,having a power supply circuit which makes available an alternatingvoltage or an alternating current of a predetermined period duration forthe supply of the gas discharge lamp with a predetermined power means,responsive to a desired reduction of a mean lamp power rating operationcompared to operation at a normal power rating, for causing an increaseof instantaneous power provided within a half period duration (T_(P)/2)immediately prior to a pole change of the alternating voltage (U_(L)) oralternating current (I_(L)).
 2. A device according to claim 1, whereinthe increase of the instantaneous power is provided up to a rated power.3. A device according to claim 1, wherein the increase of theinstantaneous power is provided to exceed the value of a rated power. 4.A device according to claim 1, wherein the increase of the instantaneouspower is provided within each half period duration (T_(P)/2) prior tothe pole change of the alternating voltage (U_(L)) or of the alternatingcurrent (I_(L)).
 5. A device according to claim 1, wherein a duration(T_(P)/2−T_(V)) of the increase of the instantaneous power is a functionof the period duration (T_(P)).
 6. A device according to claim 1,wherein the power supply circuit comprises a voltage transformer (10)having a direct voltage output and a subsequent bridge circuit (12) inwhose diagonal the gas discharge lamp (13) is disposed.
 7. A deviceaccording to claim 6, wherein a battery is provided as power source (11)and the voltage transformer (10) is configured as a directvoltage/direct voltage converter.
 8. A device according to claim 1,wherein a metal-doped high-pressure gas discharge lamp is provided asthe gas discharge lamp (13).
 9. A device according to claim 8, whereinthe device is mounted and used in a motor vehicle.